This invention relates to electronic copiers and, more particularly, to improvements in the charging device for electronic copiers.
In the electronic copier, a light-sensitive medium is precharged by a charging device prior to its exposure to a light image of an original as is well known in the art.
The prior art charging devices include corona chargers making use of the corona discharge and contact chargers having a conductive brush or a charging roller held in forced contact with the light-sensitive medium for charging the medium by contact charging. The corona charger is simple in construction and has steady performance. However, it uses a high voltage source of 5 to 10 kilovolts and, therefore, its safety and economy are inferior. Further, during the corona discharge it produces ozone and extremely deteriorates the properties of the light-sensitive medium, developer and other materials. The contact charger requires a low voltage source of 0.5 to 1 kilovolt. Also, since it is not based upon corona discharge, it is free from the generation of ozone. On the demerit side of this charger, however, it is difficult to obtain continuous uniform charging, and therefore the reliability is low. More particularly, with this charger the voltage required for the charging of the light-sensitive medium is applied in one step as a pulse voltage; that is, the maximum voltage is applied at the point of the commencement of contact between the light-sensitive medium and conductive brush or charging roller. Since in this case a sharp change of the applied voltage occurs at the commencement of contact, uniform charging of the light-sensitive medium cannot be obtained. In addition, the light-sensitive medium is likely to be electrically damaged.